The present invention relates to a method and apparatus for determining a quantity of liquid in a tank when at least one liquid sensor disposed in the tank is inoperative. More particularly, the present invention relates to a liquid-quantity sensing system in which an accurate determination of liquid quantity may be made even though one or more of the liquid-level sensors in the tank are detected as being inoperative. While the present invention will be described with respect to aircraft fuel tank sensing systems, it is to be understood that the teachings of this application are applicable to any system requiring a determination of the quantity of liquid in a tank.
Known aircraft fuel tanks come in a wide variety of shapes and sizes. Typically, aircraft fuel tanks are enclosed within the wings and fuselage of the aircraft. The wings and fuselage also contain a number of structural supports, electrical cable conduits, and other obstructions requiring the fuel tank configuration to be quite varied. Thus, a single fuel sensor in each tank will be unable to accurately measure the amount of fuel within that tank. Therefore, a plurality of such fuel sensors are usually provided in each aircraft fuel tank. When one or more of these sensors is rendered inoperative, an accurate reading of the fuel quantity in the tank is difficult to obtain.
Furthermore, the dynamics of maneuvering aircraft frequently cause the level of fuel in the tank to be other than horizontal. FIGS. 1 and 2 depict cross-sectioned and plane views of a representative aircraft fuel tank. FIGS. 1 and 2 depict a fuel tank 2, together with level sensors 4, 6, 8, and 10. When the aircraft is grounded or flying straight and level, the level of fuel will describe a horizontal line, for example, line 12 in FIG. 1. However, as the aircraft banks, the local acceleration vector 14 will move from the vertical, thus causing fuel 16 to have a level which is off-horizontal, for example, the level 12 shown in FIG. 1.
Should one of the level sensors 4, 6, 8, or 10 become inoperative, the quantity of fuel in tank 2 must be calculated based on the outputs of the remaining operative sensors. The task of making up for the inoperative sensor is exacerbated by the forces imposed on fuel 16 during aircraft maneuvering. The present invention addresses this task.
One known method for recovering fuel quantity data in a system where one sensor is lost is described in U.S. Pat. No. 4,352,159 to Colby. In Colby, when one sensor is detected as being inoperative, an output signal from that sensor is estimated based on one or more sensors ("sister sensors") corresponding to the inoperative sensor. This estimate is based on a formula providing a linear estimate of the level of fuel at the missing sensor. Colby derives the estimated sensor output by multiplying the length of the inoperative sensor by the sum of the outputs of associated sister sensors divided by the sum of the lengths of the sister sensors. For example, (with reference to FIG. 1), if sensor 6 is inoperative, and sensors 4 and 8 detect a liquid level at line 12, then the output of missing sensor 6 is estimated to be at the level 12 also. Obviously, such a rough solution is accurate only when the level of the fuel in the tank is stable and assumes that fuel quantity apportioned to the missing probe is proportional to the missing probes length as a percentage of the total length of all probes. This is an approximation required by Colby and not required by the present invention. Furthermore, the system of Colby assumes only a single attitude of fuel within the tank and thus contains a significant error component for other possible fuel attitudes. Colby's linear approximation method assumes a single, usually horizontal, fuel level. If the aircraft maneuvers and shifts the acceleration vector from the vertical position, the fuel level will change thus making the linear approximation of Colby very inaccurate. Therefore, the system of Colby is extremely inaccurate when the quantity of fuel is to be measured while the aircraft is maneuvering or where the fuel level is in an attitude other than horizontal.
A further problem with the Colby system arises where one of the sister sensors is also inoperative. In such a case, estimates for all missing sensors are unobtainable thus providing an extremely inaccurate measure of fuel quantity in the tank.
In addition, Colby assumes a fairly horizontal configuration of the fuel tank itself. Referring to FIG. 1, if sensor 8 is inoperative, Colby estimates its fuel level by a linear approximation of sensors 6 and 10. Since sensor 10 is located much higher in fuel tank 2, its reading will be very low where the fuel is horizontal. On the other hand, the level tank of fuel at sensor 6 will be quite high. Colby makes a linear approximation between a low reading of sensor 10 and a higher reading of sensor 6 to provide a mid-range reading estimate for inoperative sensor 8. Such a solution would be inaccurate since the estimate of an operative sensor 8 should also be in the area of line 12. Therefore, the Colby system is not optimum for oddly configured fuel tanks.
Yet a further disadvantage of the Colby system is the length of time required to calculate the linear approximation of fuel quantity. The Colby system requires monitoring operative sensors and then performing multiplication and division which are computer-intensive operations. In a multi-sensor system, Colby may require too much time to provide a reading of fuel in the tank.
What is needed is a liquid quantity sensing system which takes into account many different attitudes of fuel within the tank of a maneuvering aircraft, and provides accurate data when one or more sensors is inoperative. Such a system should also provide a rapid output indicating the quantity of fuel in the tank.